[Gta04-owner] [Community] OpenPhoenux Prototype of GTA04 based7 inch tablet

Dr. H. Nikolaus Schaller hns at goldelico.com
Fri Nov 9 09:33:23 CET 2012


Am 08.11.2012 um 11:30 schrieb Flemming Richter Mikkelsen:

> On Nov 8, 2012 7:50 AM, "Dr. H. Nikolaus Schaller" <hns at goldelico.com> wrote:
> >
> >
> > Am 07.11.2012 um 20:42 schrieb Flemming Richter Mikkelsen:
> >
> > > Since the display is so demanding, maybe a hardware display power
> > > switch is in order?  That way one can boot without the display on.
> >
> > Or we could simply dim the backlight by software. But it does not solve
> > the fundamental problem that the device will drain the battery even if
> > connected to a power supply while being used.
> >
> > I think nobody would expect that it should be switched off/dimmed
> > during charging and has limited operation time even if connected
> > to the power supply. I.e. it does not work in picture frame mode...
> >
> > >
> > > The backlight may be controlled by PWM, but may still demand a lot of power...
> > > I was thinking USB 3.0 could solve the power issue.  We got lots of
> > > extra space...
> > > Or maybe just a normal power connector would be better since chargers that talk
> > > USB 3.0 is not easy to find. I know I would want to charge it at work,
> > > at home and
> > > from the car...
> >
> > I now understand why even small netbooks come with a 9V 2A supply.
> >
> > > I know some low power displayes, but they are either monochrome or
> > > really slow (for ebooks) and with such a nice tablet it would be a
> > > shame not to be able to watch videos.
> >
> > Exactly. I have studied the Pixel Qi 7 inch display but it has the same
> > demand in full color mode. It appears to be a physical law that a
> > sufficiently bright 7 inch display needs 1.5 Watt or more.
> >
> > >
> > > Maybe also a bigger or double battery pack would help a little.
> >
> > Not really. It is the balance between power supply and demand.
> > Only the difference can be used to charge the battery - independently
> > of its capacity.
> 
> Sure.  I was not writing correctly.  I was thinking of battery time here... any idea about operating time with display and CPU on max pwr?  Would 2 hours be possible?
Even more. The battery we have is specified as 3900 mAh @ 3.7V. And the system appears to draw 200-400 mA for CPU, maybe +200 mA for WLAN and ~550 mA for fully illuminated backlight. This converts into ~1 A, i.e. 4 hours in full operation without power management (screen saver, suspend etc.).

And, 15 minutes ago I have had an idea. If the power supply is to weak we must simply use a stronger one. What I did not know is that it is possible to buy USB mini power supplies rated 5V, 2A. This is sufficient to operate the whole system. But running 2A through the connectors and charging unit of the GTA04 is beyond spec. It has just to be checked if they really provide 2 Amperes...

If yes, we can simply split this current between operating the display backlight and operating the GTA04 + charging the battery. This gives no more stress to the GTA04 board as if it were operated separately without a display.

This needs just 2 high power schottky diodes or maybe a little more complex circuit if we want to avoid some power loss and allow for some "soft handover". E.g. if the USB source (PC host port) is current limited and can't operate both in parallel. Then the battery must fill the gap.

Nikolaus

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